Not Applicable.
This invention relates in general to air pressure cylinders, and more particularly, to air pressure cylinders capable of operation at a broad range of atmospheric/environmental conditions, including extremely high and low temperatures.
In its most basic form, an air pressure cylinder consists of a piston, situated within a cylinder, which is connected to a push rod that exits one end of the cylinder. Ports at each end of the cylinder allow for the flow of pressurized air into the cylinder. When pressurized air is forced into a first end of the cylinder, the pressure in the cylinder forces the piston toward the second end, thereby actuating the push rod. Likewise, when air is forced into the second end of the cylinder, the air pressure forces the piston toward the first end. The push rod moves in synchronization with the piston, and it is the movement of the push rod that is harnessed for mechanical actuation purposes.
The movement of the piston against the cylinder walls creates friction. This friction escalates during continuous use and under conditions of humidity and particulate invasion. In addition, typical air cylinders are composed of metals. When these cylinders are subjected to temperature gradients, each component exhibits a complex set of thermal expansion coefficient characteristics that cause inconsistent expansion in the cylinder. These expansion inconsistencies greatly increase the friction generated in the air cylinder during operation. The friction generated in the cylinder causes wear that leads to degradation and failure. Furthermore, the cylinders can bind or lock-up during extreme temperature operation. These limitations are only exacerbated by the need in virtually all gas cylinders for some form of lubrication. When gas cylinders are used in environments with corrosive chemicals or abrasives, not only do the cylinder components degrade in a very short period of time, the lubrication will typically be compromised or destroyed rapidly by contaminants.
Air cylinders are typically classified according to the distance of piston travel. That is, if the piston in an air cylinder will have a maximum travel in one direction of 2.00 inches, the cylinder will commonly be called a xe2x80x9ctwo inchxe2x80x9d cylinder. A complete cycle of operation, in which the piston first travels completely in one direction and then in the opposite direction is considered the xe2x80x9cstrokexe2x80x9d of the cylinder. Hence, a xe2x80x9ctwo inchxe2x80x9d cylinder will have a four inch stroke. Because of this, the life of a cylinder is usually measured by the total amount of piston travel. For example, testing may show that a given two inch cylinder may operate for 500,000 strokes, on average, before failing. This equates to an expected life of 2,000,000 inches (i.e. 4 inches/strokexc3x97500,000 strokes).
The anticipated life for typical lubricated air cylinders range from approximately 3,000,000xe2x80x3 to 4,000,000xe2x80x3, under a moderate temperature and humidity range (i.e. 20xc2x0 C. to 30xc2x0 C.; at room humidity) and little or no exposure to particulate or chemical contamination. The life of typical air cylinders degrades rapidly when exposed to more extreme conditions. In fact, because of these limitations, many typical gas cylinders are designed specifically for a single, disposable use.
In many applications, it is desirable for air cylinders to operate for a much longer life. For example, a one inch air cylinder for a push punch along a non-stop assembly line may operate continuously at a rate of 100 strokes per minute. Over the period of one month, the number of strokes for that cylinder will exceed 4,000,000, or 8,000,000 inches. This means that, on average, the cylinder will need to be replaced at least twice each month to keep the assembly line operating. In addition, many applications impart more demanding conditions on the air cylinders with the same desire for long life. For example, a machine to test automobile door handles may require for each test that the test handles open and close hundreds of thousands of times in conditions that include temperature ranges between lower than xe2x88x9270xc2x0 C. and greater than 110xc2x0 C., with humidity up to 100% and controlled particulate injections. In such circumstances, cylinder failures not only cause downtime, but can disrupt or ruin ongoing testing. Some cylinders in use in testing facilities need to be replaced on a weekly basis.
Typical air cylinders are also limited in use because they are generally susceptible to a wide variety of chemical attacks. While typical cylinders can be coated or encased in chemically inert materials, and the lubricants and seals can be specially formulated to withstand specific chemicals, these measures are costly and imperfect. They impart inefficiencies in the cylinder, such as additional friction and wear at the seals, that reduce the cylinder life. Similarly, the lubricants in typical air cylinders create the potential risk of contamination in sterile or clean environment applications, such as in food handling or pharmaceutical production. To rectify this problem, a number of concepts have been adopted. For example, special lubricants have been developed for use in certain applications, and cylinders have been developed that operate without lubrication. However, each of these are also costly and imperfect. The special lubricants are more expensive, limited in application, and less effective than normal lubricants, which shortens the life of the air cylinders. The non-lubricated cylinders are more expensive and exhibit a shorter life than lubricated versions.
Finally, typical gas cylinders suffer from a problem commonly known as xe2x80x9cbottoming out.xe2x80x9d That is, when the piston is allowed to travel the full distance of the stroke under pressure, the piston will violently xe2x80x98strike the end of the inner cavity of the cylinder. Because the pistons and housings of typical gas cylinders are made of metals, this impact causes permanent distortions in the piston that can drastically shorten cylinder life or cause a catastrophic failure. Typically, users must xe2x80x9coversizexe2x80x9d their gas cylinder purchases to accommodate this problem. That is, if an application requires a 1.5 inch stroke gas cylinder, the user must purchase, for example, a 1.75 inch stroke cylinder and arrange the cylinder such that the required 1.5 inch stroke occurs in the middle of the cylinder""s 1.75 inch stroke range. This, of course, is inefficient and costly. Furthermore, any miscalculations will lead to early cylinder failures. While some typical gas cylinders incorporate cushions to soften the impact, this measure is not wholly effective and adds further cost to the cylinder.
Of course, down time in any application is undesirable. However, downtime in production is particularly expensive, since it includes maintenance and parts costs, idle time for workers, reduction in output, and increased pro rata overhead costs. Accordingly, any measure that increases the life of air cylinders without unreasonably increasing the cost of the cylinder is highly desirable.
Briefly stated, a gas cylinder of the present invention comprises a housing defining a cylindrical chamber, a piston within the housing chamber that separates the chamber into two cavities, a rod operatively connected to the piston and which extends from the piston externally of the housing; and one or more gas ports in the housing to allow the input and exhaust of gas in the cylindrical chamber. The addition of sufficient gas, through one or more of the ports, into one of the two cavities of the housing facilitates movement of the piston. Cushions of gas are formed about the piston and the rod, and the piston and rod float in these respective gas cushions,to enable the low friction movement of the piston and the rod within the cylinder housing. Hence, the cylinder can be operated without the use of a lubricant.
A piston seal is positioned about the piston. The piston seal is sized and shaped to facilitate the proper flow of gas over the piston to form the gas cushion around the piston. In one embodiment, the piston seal is generally X-shaped in cross-section. It is made from a material which is sufficiently elastic to enable the piston seal to radially expand and contract in response to changes in gas pressure in one or both chambers, thereby regulating and maintaining desired gas flow and pressure about the cylindrical walls of the piston. A radial groove about the piston holds the piston seal, and a piston seal retainer holds the piston seal in the radial groove.
In another embodiment, the piston seal comprises a pair of sealing discs spaced apart by a spacer. This seal is received in a groove having sloped side walls, to define a generally trapezoidal gap. The spacer also is trapezoidal in shape, but has a height less then the annular gap between the base of the groove and the cylinder tube, forming a gap between the outer surface of the spacer and the cylinder tube. The sealing discs slope toward each other. That is, the forward sealing disc slopes rearwardly and the rear sealing disc slopes forwardly. The groove walls, the sealing discs, and the sloped sides of the spacer all form angles with a diameter of the cylinder. The angle defined by the spacer walls is greater than the angle defined by the sealing discs; and the angle defined by the sealing discs is greater than the angle defined by the groove walls. The sealing discs seal against the cylinder tube inner wall. When air enters the cylinder, for example from a rear port, the initial blast of air flexes the rear sealing disc forwardly to xe2x80x9cunlockxe2x80x9d the seal between the sealing disc and the tube. The air the urges the forward sealing disc to into a sealing engagement with the cylinder tube.
A rod seal is also preferably positioned about the rod. The rod seal is sized, shaped, and dimensioned to facilitate the proper flow of gas about the rod sufficient to form the gas cushion about the rod to float the rod. The rod seal is elastic. In one embodiment, the rod seal is generally Vxe2x80x94or -shaped, and has a back leg or outer portion and has a flexible lip that extends from the outer portion of the seal radially inwardly toward the rod in a non-perpendicular manner. In a second embodiment, the seal is somewhat xe2x80x9cKxe2x80x9d-shaped, with the back leg of the xe2x80x9cKxe2x80x9d being angled. In both embodiments, the seal readily allows the flow of gas in one direction, but gas flow in the opposite direction flexes the seal to seal against the rod and resist the flow of gas in that opposite direction. The rod seal can be held in place about the rod by a radial groove in the housing or by a retainer in proximity with the housing about the rod. A retaining ring can also be used to hold the seal in place.
A disc at one end of the rod nearest the piston, having a radial dimension somewhat greater than the radius of the rod is loosely constrained in a well in the piston, thereby facilitating the general synchronous movement of the rod and the piston while allowing both to independently float on gases in the cylinder.
In addition, because the housing and piston components are all made of an elastic material, the present invention is able to withstand full pressure, full-stroke impacts without suffering significant loss of cylinder life.
In one embodiment, the cylinder tube, end caps, piston, and piston and rod seals are uniformly composed of a high density plastic, having a molecular weight greater than 500,000 and a coefficient of sliding friction lower than 2.0. These material properties, in conjunction with the novel design disclosed herein, enable the gas cylinder apparatus to operate for extended periods of time over a broad range of temperatures, at least including all temperatures between xe2x88x9260xc2x0 C. to 110xc2x0 C., with exposure to numerous corrosive and abrasive contaminants, and without lubrication. In another embodiment, the end caps are made from a metal, such as aluminum or stainless steel, while the remaining noted components (i.e., the cylinder tube, the piston, and piston and rod seals) are made from the high density plastic.